Формула

$\displaystyle \left({\frac {d}{dt}}\right)^{m}u(x,t)=\sum _{j=0}^{m-1}\alpha _{j}(x,t;{\frac {d}{dx}})\left({\frac {d}{dt}}\right)^{j}u(x,t)+f(x,t) \qquad$